ON THE PULSE-PERIOD EVOLUTION OF THE X-RAY PULSAR 1E-2259+586

Precession of spin motion is considered for rapidly rotating massive w hite dwarfs. We suggest that the X-ray pulsar 1E 2259+586 may be such an object. A modulation of spin period due to precessional motion of s pin axis about the axis of symmetry may explain the change in period. It is shown that this may be responsible for the decrease in the mean spin-down rate which was observed for 1E 2259+586 between the year 198 7 and 1990. The low-mass X-ray binary models for the pulsar have also been outlined. One finds that a number of observations of 1E 2259+586 may be explained in the framework of this model although the companion is uncertain. Thus, the present data are not sufficient enough to unw arrant the discussion of either of the two models.